Use LightDM for display management

Advantages over GDM:
- Faster - the greeter doesn't require an entire GNOME session to run
- More flexible - multiple greeters are supported through a well defined interface. This allows Ubuntu derivatives to use the same display manager (e.g. Kubuntu, Lubuntu etc).
- Simpler codebase - similar feature set in ~5,000 lines of code compared to 50,000 in GDM
- Supports more usecases - first class support for XDMCP and multihead.